1. Field of the Invention
This invention relates to a glass reflector coated on both sides with an optical interference film. More particularly this invention relates to all glass reflectors and their use with lamps, wherein both the inside and the outside surfaces of the reflector are coated with an optical interference film deposited by a low pressure chemical vapor deposition process.
2. Background of the Disclosure
Thin film optical interference coatings known as interference filters or optical interference films which comprise alternating layers of two or more materials of different refractive index are well known to those skilled in the art. Such coatings or films are used to selectively reflect and/or transmit light radiation from various portions of the electromagnetic spectrum such as ultraviolet, visible and infrared radiation. These films or coatings are used in the lamp industry to coat reflectors and lamp envelopes. One application in which these coatings have been found to be useful is to improve the illumination efficiency or efficacy of incandescent and arc lamps by reflecting infrared radiation emitted by a filament or arc back to the filament or arc while transmitting the visible light portion of the electromagnetic spectrum emitted by the filament or arc. This lowers the amount of electrical energy required to be supplied to the filament or arc to maintain its operating temperature. Such films have also been applied to reflectors in the form of what is known in the art as cold mirrors. A cold mirror in the prior art is a glass or plastic reflector coated on the inside reflecting surface with an optical filter which reflects visible light thereby projecting it forward of the reflector, while at the same time permitting longer wavelength infrared energy to pass through the coating and the reflector. This insures that the light projected forward by the reflector is much cooler than it would otherwise be if both the visible and the infrared light were reflected and projected forward. On the other hand, some reflectors contain a completely reflecting coating on the inside reflecting surface, such as aluminum or optical interference coating, for reflecting all of the radiation emitted by the lamp filament or arc and projecting same forward of the reflector. In this latter case, the projected light is significantly hotter than that obtained with a cold mirror.
One of the problems that has been encountered results from the processes, such as vacuum sputtering and reactive plasma or electron beam evaporation, employed to coat reflectors. In these types of processes, it is difficult and sometimes impossible to coat articles having complex shapes, because these processes are line-of-sight processes or approximately line-of-sight processes. With all glass reflectors having a rearwardly protruding socket cavity or nose portion into which a lamp is cemented, these prior art coating processes have been unable to coat the surface of the cavity and this has resulted in a significant amount of bright, white visible light being projected out through the rear socket portion of the reflector.